1. Field of the Invention
This invention relates to a stencil printer system provided with a stencil making section, and more particularly to an improved structure for applying a back tension to the stencil material when a stencil made by the stencil making section is wound around the printing drum.
2. Description of the Related Art
There has been known a stencil printer system with a stencil making section. In such a stencil printer system, there has been known a technique in which, in order to associate stencil making with winding the stencil around the printing drum, a stencil conveyor roller and a stencil discharge roller are provided downstream of the stencil making section. When a stencil is being made, the conveyor roller is driven while the discharge roller is stopped, whereby the stencil is held slackened between the stencil conveyor roller and the stencil discharge roller until the stencil is finished. After the stencil is finished, the printing drum is rotated with the leading end of the stencil clamped by a clamper on the printing drum while the stencil discharge roller is driven by the printing drum by way of the stencil applying a tension to the stencil by means of a one-way clutch built in the stencil discharge roller. See, for instance, Japanese Patent No. 2538817.
When winding the stencil around the printing drum, it is important to apply a stable back tension to the stencil in order to properly winding the stencil around the printing drum. However, when the back tension is applied to the stencil only by driving the stencil discharge roller by way of the stencil as in the prior art, the back tension applied to the stencil cannot be stabilized.
That is, in the state after the stencil is made and before it is wound around the printing drum, the stencil is held slackened between the stencil conveyor roller and the stencil discharge roller and the stencil discharge roller is small in diameter and in a line contact with the stencil. Accordingly, until the stencil charge roller starts to be rotated pulled by the stencil, a certain back tension can be applied to the stencil. However, after the stencil discharge roller once starts rotating, the tension applied to the stencil fluctuates with rotation of the stencil discharge roller.
This problem may be overcome by providing a tension blade which is pressed against the stencil on the stencil conveyance path between the stencil material roll and the printing drum.
However, when such a tension blade is not accurately positioned with respect to the stencil, the expected result cannot be obtained. That is, in order to apply a desired back tension to the stencil in a predetermined tension distribution in the transverse direction of the stencil, it is required that the shape of the tension blade and the tension blade mounting accuracy are high in all the directions, which makes it very difficult to realize this method of applying a proper back tension to the stencil.
For example, when the tension blade is obliquely pressed against the stencil, the stencil runs obliquely, whereby the stencil can be wrinkled or the stencil cannot be wound around the printing drum in the regular position, which can result in unsatisfactory printings. Further, when the tension blade is not pressed against the stencil at a uniform pressure, the stencil undergoes force which tends to displace the stencil toward a position and wrinkle the stencil.
A tension blade high in dimensional accuracy is difficult to manufacture and to mount the tension blade with high mounting accuracy requires accurate alignment in all the directions, which makes troublesome the maintenance of the printer system.
In view of the foregoing observations and description, the primary object of the present invention is to provide a stencil printer system in which the stencil can be accurately wound around the printing drum in a desired position without fear that the stencil is obliquely wound around the printing drum or the stencil is wrinkled and at the same time, the requirements of the dimensional accuracy and the mounting accuracy of the tension blade are relaxed.
In accordance with the present invention, there is provided a stencil printer system in which a stencil is made by the use of master material unrolled from a stencil material roll and is wound around a printing drum by clamping the leading end of the stencil on the printing drum and rotating the printing drum while the stencil is once slackened on the way to the printing drum, wherein the improvement comprises that
there is provided along the stencil conveyance path between the stencil material roll and the printing drum a tension blade which is pressed against the stencil to apply back tension to the stencil when the stencil is run by the printing drum and the tension blade is formed so that the stencil undergoes force which tends to displace outward the stencil from the central portion in the direction of width of the stencil.
For example, the tension blade may be formed so that the stencil undergoes force which tends to displace outward the stencil from the central portion in the direction of width of the stencil by forming grooves on the surface of the tension blade to be brought into contact with the stencil so that the contact area of the tension blade with the stencil is smaller at the outer portion than at the central portion of the stencil. In this case, it is preferred that the grooves be formed so that the contact area of the tension blade with the stencil becomes gradually smaller toward the outer portion from the central portion of the stencil.
Further, the tension blade may be formed so that the stencil undergoes force which tends to displace outward the stencil from the central portion in the direction of width of the stencil by forming grooves on the surface of the tension blade to be brought into contact with the stencil to extend obliquely outward forward of the direction of conveyance of the stencil. In this case, the grooves may be formed so that the contact area of the tension blade with the stencil is smaller at the outer portion than at the central portion of the stencil.
It is preferred that the tension blade be symmetrical with respect to the longitudinal axis of the stencil.
In accordance with the present invention, since back tension is applied to the stencil so that the stencil undergoes force which tends to displace outward the stencil from the central portion in the direction of width of the stencil, back tension can be stably applied to the stencil without fear that the stencil is obliquely wound around the printing drum or the stencil is wrinkled. At the same time, the requirements of the dimensional accuracy and the mounting accuracy of the tension blade are relaxed, whereby manufacture and mounting of the tension blade are facilitated, which is advantageous from the viewpoints of cost and maintenance. These advantages are not lost by change with time of the tension blade.
When tension blade is formed with grooves on the surface of the tension blade to be brought into contact with the stencil so that the contact area of the tension blade with the stencil is smaller at the outer portion than at the central portion of the stencil and/or so that the grooves extend obliquely outward forward of the direction of conveyance of the stencil, force which tends to spread outward the stencil acts on the stencil as the stencil is run, whereby longitudinal wrinkle parallel to the direction of conveyance of the stencil is prevented from being generated and the back tension applied to the stencil is stabilized.